Nonwoven fabrics or webs have a structure of individual fibers or threads which are interlaid, but not in a regular or identifiable manner as in a woven fabric. Nonwoven fabrics or webs have been formed from many processes which include meltblowing, spunbonding and air laying processes. The basis weight of fabrics is usually expressed in grams per square meter.
Nonwoven spunbonded fabrics are used in many applications and account for the majority of products produced or used in North America. Almost all such applications require a lightweight disposable fabric. Therefore, most spunbonded fabrics are designed for single use generally requiring minimum bond strength and are designed to have adequate properties for the applications for which they are intended. Spunbonding refers to a process where the fibers, filaments, are extruded, cooled, and drawn and subsequently collected on a moving belt to form a fabric. The web thus collected is not bonded and the filaments must be bonded together thermally, mechanically or chemically to form a fabric. Thermal bonding is by far the most efficient and economical means for forming a fabric. Hydroentangling is not as efficient, but leads to a much more flexible and normally stronger fabric when compared to thermally bonded fabrics. Thermal bonding is one of the most widely used bonding technologies in the nonwovens industry. It is used extensively in spunbond, meltblown, air-lay, and wet-lay manufacturing as well as with carded-web formation technologies. Considerable effort has been spent on trying to optimize the web-formation processes, bonding processes, and the feed fiber properties to achieve the desired end-use properties while reducing the cost of manufacture. One way to reduce the cost of manufacture is to produce more nonwoven fabric on the same machine by processing faster. It has been found that satisfactory bonds can be made faster at higher temperatures, up to a point, after which satisfactory bonds can no longer be made. This is sometimes described as “the bonding window closes as the bonding temperature increases”. The processing window at a given process speed is defined by the maximum and minimum process temperatures that produce nonwovens with acceptable properties. In other words, it has been found that as one attempts to process faster, the difference between the maximum and minimum process temperatures gets smaller until they merge into a single temperature. At still higher speeds, no suitable nonwoven can be made, regardless of the bonding temperature, i.e. the processing window closes.
In addition, over the last 100 years of modern fiber science, it has been learned that stronger fibers generally make stronger textile structures when all the other construction factors are the same. This applies to cords, ropes, knits and wovens. In addition, for melt-spun fibers, it is possible to make stronger fibers by increasing fiber orientation and crystallinity, as well as achieving appropriate fibrillar morphology. This is typically accomplished by increasing the spinning speed, altering the quenching conditions, increasing the draw ratio and annealing the fibers under tension. Hence, it was unexpected when it was found that thermally point bonded nonwoven fabrics became weaker when high strength fibers were used and, conversely, yielded stronger fabrics with appropriate weaker fibers.
Part of the confusion about the strength of nonwovens can be attributed to the fact that the failure mode changes with bonding conditions. It has been observed that the strength of the bonded fabric increases with bonding temperature or with bonding time up to a point, and then the bonded fabric strength begins to decrease. For bonding conditions below this peak, failure occurs by bond disruption, i.e. the bond simply pulls apart. Above the peak, failure occurs by fiber breakage at the bond periphery. Several explanations for this latter observation have been provided. One explanation that has been forwarded is that there is a stress concentration at the bond periphery, where most failures occur. Although this is likely to be true, no satisfactory explanation of the dependence of the stress concentration on bonding conditions has been provided. Another proposed failure mechanism is that the fibers are crushed by the calendar rolls and thus weakened at the bond edge where the edges of the bond point flatten the fibers. However, as shown by Chidambaram, A., Davis, H., Batra, S. K., “Strength Loss in Thermally Bonded Polypropylene Fibers” Inter Nonwovens J 2000, 9(3) 27 this factor accounted for only a small portion of the loss of strength. Furthermore, bond strength did not correlate with bonding pressure, as one would expect for this failure mechanism. To date, no satisfactory explanation of the mechanical failure mechanisms of thermally point bonded nonwovens has been provided.
Thermal bonding can be performed in several ways. In through-air bonding, a hot fluid, air, is forced through a preformed web. If the temperature of the fluid is high enough, the fibers may become tacky and adhere to one another. In this case they form bonds where two or more fibers come into contact. In infrared bonding, IR-bonding, infrared light provides the heat. In ultrasonic bonding, friction between contacting fibers due to the application of ultrasound causes the fibers to become tacky and bond. In thermal point bonding, the preformed fiber web is passed between heated calendar rolls. The rolls may be smooth or embossed with a bonding pattern. A uniform fabric requires uniform pressure, uniform temperature and uniform input web. Bonding occurs only where the fibers contact the heated rolls. Therefore, on a smooth calendar roll, bonding occurs wherever fibers cross each other while on an embossed calendar roll, bonding occurs primarily between the raised areas. This results in bonding “points” or “spots”. In each of these processes, the underlying physics is the same, the fibers are heated, they form a bond, and they are subsequently cooled.
Before bonding can occur, a web must be formed. The processes usually employed include spinning (spunbond), melt-blowing, wet-laying, air-laying and carding. Each of these produces different fiber orientation distribution functions (ODF) and web densities. It is important to recognize that there is an interaction between the web structure and the efficiency with which bonds are formed, i.e. bonding efficiency. In the simplest case where smooth calendar rolls are used, or in through-air bonding, the maximum level of bonding occurs when the structure is random since the maximum number of fiber-to-fiber crossovers is achieved. Thus, the more oriented the structure, the fewer the number of potential bond sites. The ODF also dictates, to a great extent, the manner in which the structure undergoes mechanical failure. While failure can follow different modes, the fabrics tend to fail by tearing across the preferred fiber direction when the load is applied parallel to the machine- or cross-directions. At all other test angles, failure is likely to be dictated by shear along the preferred direction of fiber orientation.
It is generally observed that the strength of the structure improves with bonding temperature, reaches a maximum, and then declines rapidly because of over-bonding and premature failure of the fibers at the fiber-bond interface. However, regardless of the bonding temperature, the changes brought about in the web structure and the microscopic deformations therein are driven by the initial ODF of the fibers, and therefore are similar for all structures with the same initial ODF. During load-elongation experiments the nature of the bonding process controls the point at which the structure fails, but the behavior up to that point is dictated by the structure (ODF) and the anisotropy of the bond pattern. Moreover, the structure stiffness, i.e. tensile modulus, bending rigidity and shear modulus, continues to increase with bonding temperature.
After the web is formed, it passes through the calendar rolls where it is bonded. Thermal point bonding proceeds through three stages: 1) compressing and heating a portion of the web, 2) bonding a portion of the web, and 3) cooling the bonded web. In calendar bonding, the bonding pressure appears to have little or no effect on fabric performance beyond a certain minimum. This is especially true for thin nonwovens where minimal pressure is required at the nip to bring about fiber-to-fiber contact. Sufficient pressure is needed to compact the web so that efficient heat transfer through conduction can take place. In addition, pressure aids plastic flow at elevated temperatures, thereby increasing contact area between the fibers as well as decreasing thickness at the bond even further. Pressure also aids “wetting” of the surfaces. This requires fairly minimal pressures. Pressure also constrains the mobility of the fibers in the bond spot. Over the range of pressures commercially employed, higher nip pressures do not necessarily lead to higher performance.
In calendar and through-air bonding, it is quite easy to obtain under-bonded or over-bonded structures. Under-bonding occurs when there are an insufficient number of chain ends in the tacky state at the interface between the two crossing fibers or there is insufficient time for them to diffuse across the interface to entangle with chains in the other fiber. The formation of a bond requires partial melting of the crystals to permit chain relaxation and diffusion. If, during bonding, the calendar roll temperatures are too low or if the roll speeds are too high, the polymer in the mid-plane of the web does not reach a high enough temperature to release a sufficient number of chains or long enough chain segments from the crystalline regions. Thus, there will be very few chains spanning the fiber-fiber interface, the bond itself will be weak, and the bonds can be easily pulled out or ruptured under load, as observed.
Over-bonding occurs when many chains have diffused across the interface and a solid, strong bond has been formed. The fibers within the bond spot, and at the bond fiber periphery, have lost their orientation and their strength, but the bond spot itself represents a more rigid and larger area compared to the fibers entering the bond spot. However, at the same time, the polymer chains within the fibers located in the vicinity of the bond, also relax to lower birefringence as heat diffuses along the fiber length. Thus the fibers entering the bond have also lost some of their molecular orientation and consequently their strength at the fiber-bond interface. The distance that sufficient heat diffuses along the fiber length subjected to heating depends on the time and temperature in the nip. It has been observed at high speeds, this distance should be less than the thickness of the nip, while at lower speeds the distance should be longer. Since the birefringence is only reduced where the temperature was high enough to start melting the crystals, it is only this region that has reduced strength. Thus the birefringence of the fibers is reduced only in the region close to the bond periphery and the fibers are weak only in this region. They may have also become flat and irregular in shape. The bond site edge becomes a stress concentration point where the now weaker fibers enter. In a fabric under load, this mechanical mismatch results in the premature failure of the fibers at the bond periphery, as observed. Simply put, over-bonding occurs when too much melting has occurred.
Thermal bonding of nonwoven webs occurs through three steps 1) heating the fibers in the web, 2) forming a bond through reptation of the polymer chains across the fiber-fiber interface, 3) cooling and resolidifying the fibers. In calendar bonding, step 1 must occur while the web is in the nip. Step 2 must begin while the web is in the nip to tie the structure together, but it can finish during the initial portion of step 3. There is excellent agreement between the required times for heating and forming the bond and commercial bonding times.
In under-bonded webs, there are too few polymer chains diffusing across the fiber-fiber interface. During tensile testing, these bonds simply disintegrate. In well-bonded webs there is sufficient diffusion of the chains across the interface to form a strong bond, but only a moderate loss of mechanical properties of the bridging fibers at the bond periphery. Hence there is an acceptable trade off between the strength of the bond and the strength of the fibers at the bond periphery. In over-bonded webs, there is sufficient diffusion of the chains across the interface to form a strong bond, but there is a large loss of mechanical properties of the bridging fibers at the bond periphery. During tensile testing, the fibers break at the bond periphery.
Hydroentangling results in somewhat different characteristics. The bonded fibers will be flexible and will have a higher strength than its calendar bonded counter part. The fabric does not go through shear failure as easily as thermally point bonded nonwovens.
Bicomponent nonwoven filaments are known in the art generally as thermoplastic filaments which employ at least two different polymers combined together in a heterogeneous fashion. Most commercially available bicomponent fibers are configured in a sheath/core, side-by-side or eccentric sheath/core arrangement. Instead of being homogeneously blended, two polymers may, for instance, be combined in a side-by-side configuration so that a first side of a filament is composed of a first polymer “A” and a second side of the filament is composed of a second polymer “B. Alternatively, the polymers may be combined in a sheath-core configuration wherein the outer sheath layer of a filament is composed of first polymer “A” and the inner core is composed of a second polymer “B”.
Bicomponent fibers or filaments offer a combination of desired properties. For instance, certain resins are strong but not soft whereas others are soft but not strong. By combining the resins in a bicomponent filament, a blend of the characteristics may be achieved. For instance, when the bicomponent fibers are in a side-by-side arrangement these are usually used as self-bulking fibers. Self-bulking is created by two polymers within a filament having a different strain level or shrinkage propensity. Hence, during quenching or drawing they become crimped. Also, for some sheath/core configurations, the polymer utilized for the sheath component may have a lower melting point temperature than the core component. The outer component sheath component is heated to become tacky forming bonds with other adjacent fibers.
An additional bicomponent fiber is known as an islands-in-sea fiber. In such a configuration, a “sea” component forms the sheath, with the “island” components being the core or cores. Typically, islands-in-sea fibers are manufactured in order to produce fine fibers. The production of nanofibers in and of themselves is infeasible with current technology. Certain fiber size is necessary to insure controlled manufacturing. Accordingly, to produce nanofibers, islands-in-sea fibers consist of a sea component which is soluable and when removed results in the interior fibers being released. Also, it is known in some circumstances to maintain the sea component. U.S. Pat. No. 6,465,094 discloses a specific fiber construction which is of an islands-in-sea type configuration wherein the sheath, e.g. sea, is maintained to provide the fiber with distinct properties. Such a structure is akin to a typical bicomponent sheath/core construction with multi cores enabling certain fiber properties to be created.
While prior art bicomponent fibers are known, there is a need for a high strength, lightweight nonwoven fabric.
In view of the aforementioned, it is an object of the present invention to provide a method for producing high strength spunbonded nonwoven fabrics;
It is a further object of the present invention to establish a fiber construction which is bonded in a manner which enables the fiber to exhibit high tensile and tear strength characteristics previously unfounded in nonwoven fabrics.